1. Field of the Invention
The present invention relates to surface acoustic wave apparatuses and communication apparatuses, and more particularly, to a surface acoustic wave apparatus for use in, for example, a surface acoustic wave multiplexer and a communication apparatus including such a surface acoustic wave apparatus.
2. Description of the Related Art
Surface acoustic wave apparatuses are provided with a surface acoustic wave device which utilizes a surface acoustic wave propagating along a surface of a piezoelectric member, and are used for delay lines, filters, resonators, and other apparatuses. Since surface acoustic waves have shorter wavelengths than electromagnetic waves, it is easy to reduce the size of surface acoustic wave apparatuses. Therefore, surface acoustic wave apparatuses are used for filters provided in high-frequency circuits, for example, in portable telephones.
Recently, mobile-communication units such as portable telephones have been required to be made more compact and to have a lower profile. Therefore, surface acoustic wave apparatuses must also be made more compact and to have a lower profile. In portable telephones, to perform transmission and receiving at different frequency bands with the use of one antenna, surface acoustic wave apparatuses have been increasingly used as multiplexers (branch filters).
An example of a surface acoustic wave apparatus used for a surface acoustic wave multiplexer is disclosed in (i) Japanese Unexamined Patent Publication No. 05-167388. In this technology, as shown in FIG. 17, a plurality of surface acoustic wave resonators (hereinafter referred to simply as resonators) is provided to define ladder-type surface acoustic wave filters (hereinafter referred to as filters) and two of such filters are connected in parallel to define a multiplexer.
More specifically, in the multiplexer, ladder-type filters Fi and Fii each formed by alternately connecting series resonators S and a parallel resonator P are provided, and the filters are connected in parallel at a common terminal To. The filter Fii has higher pass-band frequencies than the filter Fi. If the multiplexer is used for a communication apparatus, the filter Fii is used as a receiving filter and the filter Fi is used as a transmission filter.
When two surface acoustic wave filters define a multiplexer as described above, the following filter characteristic (impedance characteristic) is required for the filter Fi. The filter Fi must have an impedance that is close to that of the entire circuit in the pass band and a much higher impedance than that of the entire circuit in a blocking band, which is the pass band of the filter Fii. Since it is not easy for conventional transversal surface acoustic wave filters to have such a filter characteristic, the circuit structure of the entire multiplexer is complicated.
In contrast, in the device disclosed in the above-described publication (i), since resonators Si and Sii that are disposed closest to the common terminal are series resonators connected in series, the series resonators are used to match the impedance characteristics of the filters and also for phase adjustment in the entire multiplexer. Therefore, the impedance is much higher than the circuit impedance in the blocking bands, other than the pass band, to achieve the required impedance characteristic.
In the multiplexer having the above-described structure, in the pass band of one filter, the other filter functions as a capacitive device connected in parallel. When an inductance device L having an inductance to cancel the capacitance of the capacitive device (the other filter) is connected in parallel close to the common terminal To, as shown in FIG. 17, the capacitive-device function of the other filter is canceled and matching is provided, for example, at 50Ω.
An example of a filter defining a parallel capacitive device, as described above, used for a multi-terminal-pair surface acoustic wave filter is disclosed in (ii) Japanese Unexamined Patent Publication No. 10-313229. In this filter, an inductance device is connected in parallel close to a common terminal for matching. In addition, the surface acoustic wave filter is designed so as to be parallel-resonant with the parallel inductance device. Only a single inductance device is provided as a matching circuit to prevent an effect of the other filter.
In the above-described conventional devices, the above-described matching is obtained theoretically. When such a surface acoustic wave multiplexer is integrated into one package for compactness and mounted as a device, however, matching cannot be sufficiently made due to a series parasitic component.
Specifically, when a multiplexer having the above-described structure is integrated into one package and mounted, as shown in FIG. 18, a series parasitic impedance component Z0 caused by striplines connected to the filters Fi and Fii is formed between the signal terminals of the filters Fi and Fii and a matching circuit (parallel inductance device L) at the common-terminal side. If such a parasitic component is included, the impedance of the multiplexer is shifted toward a lower-impedance side. Matching cannot be achieved with only the inductance device L being provided.
More specifically, it is assumed that matching is made at 50Ω in the pass band fi of the filter Fi without additionally providing the filter Fii or the matching circuit. Since the filter Fii functions as a capacitive device in its pass band fii, the impedance characteristic of the filters Fi and Fii, viewed from a connection point P0 where the filters are connected, is obtained by the filter Fi having a matching of 50Ω and a parallel capacitor.
In an admittance chart, the admittance of the filter Fi is changed from a point A to a point B, as shown in FIG. 19. In the admittance chart, since an upper semi-circle indicates an inductive admittance, and a lower semicircle indicates a capacitive admittance, the admittance of the filter Fi is capacitive on an equal-conductance circle.
Therefore, if the above-described series parasitic impedance component Z0 is not present between the connection point P0 and the inductance device L, matching could be made at 50Ω in the filter Fi by providing the inductance device L only. As a result, in the admittance chart shown in FIG. 19, the admittance of the filter Fi is changed from the point B to the point A by setting the inductance of the inductance device L to an appropriate value.
Actually, the above-described series parasitic impedance component Z0 is present between the filter Fi and the inductance device L, as shown in FIG. 18. Therefore, the phase is shifted toward a lower impedance side. In the admittance chart shown in FIG. 19, the admittance of the filter Fi is changed from the point B to a point C on an equal-susceptance circle D. Consequently, matching cannot be achieved at 50Ω by providing only the inductance device L. The admittance of the filter Fi can be changed only on the equal-susceptance circle D.
To solve the foregoing problem caused by the parasitic component, it is necessary to provide, in addition to the parallel inductance device L, another matching device, such as a series capacitive device or a series stripline, as a matching circuit. As a result, the number of matching devices is increased, and the size of the multiplexer is increased.